1. Field of the Invention
The present invention relates to a back projection type screen having a large aspect ratio and capable of displaying an image with improved uniformity of brightness. The present invention relates also to a back projection type image display apparatus using the screen.
2. Related Background Art
A back projection type image display apparatus is known in the art. FIG. 1 illustrates a typical example of the back projection display apparatus according to the prior art. In order to show the essential components thereof, the cabinet and other auxiliary parts are all omitted in FIG. 1.
The display apparatus shown in FIG. 1 comprises a display device 1, a projection lens 2, a Fresnel lens sheet 8 and a lenticular lens sheet 9. The display device 1 may be, for example, a cathode ray tube (CRT). The Fresnel lens sheet 8 has a Fresnel lens formed on it serving as a condenser lens. The lenticular lens sheet 9 is provided for the control of the angle of field. If desired, a diffusing agent may be incorporated into the lenticular lens sheet. Also, a diffusing plate may be provided in stead of or in addition to the lenticular lens sheet 9. The Fresnel lens 8 is of the concentric form which is generally used at present. In the display apparatus shown in FIG. 1, an enlarged image light from the projection lens 2 is incident on the Fresnel lens sheet 8 perpendicularly (at the center of the sheet). But, there is known and used another type of display apparatus in which the enlarged image light from the projection lens obliquely enters onto a screen as shown in FIG. 2.
Referring to FIG. 2, an enlarged image light from the projection lens 2 is reflected by mirrors 3 and 4 and then incident on a screen 10. The angle of incident is .theta..sub.o. Reference numeral 5 denotes a cabinet.
The first-mentioned apparatus as shown in FIG. 1 is referred to as a perpendicular incidence type of display in which .theta..sub.o =0. The second-mentioned one is referred to as an oblique incidence type of display. As readily seen in FIG. 2, the arrangement of the oblique incidence type has a particular advantage in practice that the depth l of the cabinet 5 can be reduced to a great extent as compared with that of the perpendicular incidence type. However, it involves some problems. Firstly, since the image light toward the viewer from the screen 10 is deviated down from the horizontal direction by an angle .theta..sub.o, the picture plane appears somewhat dark for the viewer. Secondly, since the angle of incidence .theta..sub.o at the lower portion of the screen becomes large, an amount of reflection loss is caused at the surface of the Fresnel lens constituting the screen. Also, there occurs eclipse of light beam. As a solution to these problems, it has already been proposed to dispose plural eccentric Fresnel lenses on the inner side (on the image light incidence side) of the screen 10 so as to refract the image light in the horizontal direction, and also known to use a power-dispersed eccentric Fresnel lens which is able to moderately and gradually refract and deflect even such image light having a large incident angle.
However, in the prior art display screen, a concentric Fresnel lens as shown above is used for both of the perpendicular incidence type and the oblique incidence type, which necessarily leads to the problem of deterioration of brightness on the screen. The problem is enhanced in the case of high definition television (HDTV) whose screen is longer in horizontal and has a larger aspect ratio than that of the conventional NTSC system of image display apparatus. On the screen of HDTV, the brightness of image light decreases remarkedly at the marginal areas along the left and right sides of the screen. In the case of conventional NTSC system in which an aspect ratio of 3:4 is used, the ratio of the brightness at the upper and lower marginal area and the brightness at the left and right marginal area (which is referred to as brightness difference) is not so large as to bring about a trouble. In contrast, the aspect ratio of the modern HDTV system is 9: 16 and the brightness difference is 1.8 folds (.about.16/9) which is far larger than 1.3 folds (.intg.4/3) of the conventional system. Such a large brightness difference is no longer negligible.
Referring to FIG. 3, an image of the CRT 1 is formed on the screen 10 through the projection lens 2.
It is most desirable that the brightness of the image is uniform over the screen 10. However, in practice, the brightness decreases down with the distance away from the optical axis 0. More specifically, the brightness of off-axial image decreases in proportion to the fourth power of the cosine of .omega. (rule of 4th power of cosine), wherein .omega. is the inclination angle of the principal ray incident upon the optical system relative to the optical axis. This decrease in brightness occurs even when the area of the entrance pupil of the projection lens 2 is kept constant.
For an oblong screen, therefore, the inclination angle .omega. becomes larger at the marginal area along the left and right sides of the screen. Consequently, according to the rule of 4th power of cosine, the brightness at the marginal area along the left and right sides is lower than the brightness at the marginal area along the upper and lower sides of the screen.
Thus, when a horizontally long and large screen is used, there is inevitably the problem of a decrease of brightness at the area along the left and right sides of the screen regardless of whether it is the perpendicular incidence type or the oblique incidence type.